Gold alloys have been deposited for many years onto watchcases, watchbands, eyeglass frames, writing instruments, costume jewelry, and the like. The karat of these deposits usually ranges from 12 to 18, the deposit thicknesses range from 2 to 20 microns, and the deposit colors are pale yellow to pink.
For many years, the most often utilized electroplated gold alloy for these applications has been gold-copper-cadmium. Since cadmium is such a o poisonous metal, however, the electroplating industry has been searching for a substitute having a reduced level of toxicity. In addition to being non-toxic, the gold alloy deposits produced with such a cadmium substitute must have the following required physical characteristics:
1. The deposits must have the correct color, as required. Usually, these colors are the Swiss standard "1-5N", which range from specific pale yellow to pink gold alloys, with the "2N" yellow grade being preferred. PA1 2. The deposits must be bright so that no further polishing is required after plating. This degree of brightness must be maintained even for thick deposits as high as 20 microns. PA1 3. The plating bath must produce deposits that exhibit levelling such that tiny imperfections in the basis metal are smoothed out or covered. PA1 4. The karat of the deposits should be as required. These karats generally range from about 12 to 18, or about 50-75% gold. PA1 5. All deposits must be reasonably ductile and capable of passing the required ductility tests, even with thicknesses as high as 20 microns. PA1 6. The deposits should be corrosion resistant and capable of passing the required corrosion tests.
A number of attempts have been made in the past as described below to deposit gold-copper-silver alloys as a substitute for the conventional gold-copper-cadmium alloys in a manner which can readily meet all of the above requirements.
U.S. Pat. No. 5,006,208 to Kuhn et al. discloses a gold-copper-silver alloy deposit from a cyanide solution using a selenocyanate brightener in an amount of 0.1 to 1 mg/liter. However, whereas deposits of 2-3 microns of gold-copper-silver alloys deposited as described by the reference were determined to provide satisfactory performance, thicker deposits of 10-20 microns, were found not to be sufficiently bright to enable commercial use of this process. In addition, the leveling characteristics of these processes were also insufficient. Moreover, the '208 patent additionally describes, e.g., at col. 1, lines 43-62, a variety of other prior art techniques for depositing gold-copper-silver alloys which were likewise found not to meet the requirements set forth above.
Japanese Patent Publication No. 62-164890, published Jul. 21, 1987 also discloses the deposition of gold-copper-silver alloys from cyanide solutions. In these plating solutions potassium citrate and a non-ionic surfactant were included as additives. This process was also found to perform unsatisfactorily when thicker deposits were attempted in that such deposits lacked brightness and were insufficiently leveled.
None of the disclosures discussed above have resulted in a commercially acceptable plating bath. That is, they have not been shown to be capable of producing deposits with the required characteristics set forth above. Thus, the relatively undesirable gold-copper-cadmium alloys are still in wide commercial use as of the present date since, until the present invention, there has been no commercially acceptable substitute.